Using Interlocked-Breakers For Maintainable Automatic Power Transfer

Space limitations forced a design-build company to use a novel approach to critical load transfer without interruption. Continuous facility operation drove the use of a very detailed sequence of construction to replace existing distribution equipment.Floor space for new electrical equipment always seems to be scarce in buildings, especially those undergoing renovation. We hit this problem head on

James D. Martin | Jan 01, 1998

Space limitations forced a design-build company to use a novel approach to critical load transfer without interruption. Continuous facility operation drove the use of a very detailed sequence of construction to replace existing distribution equipment.

Floor space for new electrical equipment always seems to be scarce in buildings, especially those undergoing renovation. We hit this problem head on in a recent upgrade of one of our own design-build projects completed some 20 years ago. The new project, located at First Security Service Company's (FSSC's) Central Operations Building in Salt Lake City, involved electrical equipment replacement in an existing electrical room built in 1976. The basis of the new project was improved electrical system performance and reliability.

Part of the renovation was to replace existing non-bypass, non-isolation automatic transfer switches (ATSs) with drawout-style transfer equipment that could be maintained without shutdown.

Problem: The existing switchboard enclosure wasn't large enough to be retroffitted with this type of transfer device.

The solution: Use freestanding bypass ATS units and install them in an old 400Hz UPS room, which is adjacent to the main switchboard room.

Other distribution equipment also needed attention We recommended the owner replace all circuit breakers in the existing switchboards with drawout-style breakers.

Another problem: The existing 400Hz UPS room was just barely big enough to house the new drawout switchgear, even with the dividing wall removed. (As shown in Fig. 3 (in the original article's page 56, the wall separated the existing 60-Hz electrical room from the existing 400-Hz electrical room.) With the new switchboards in place, we had only a few in. of clearance between the new board and existing board. There simply wasn't enough space available to install freestanding, bypass, isolation ATS units. They would have added at least 8 ft to each switchboard, for a total of 16 ft to each lineup.

What we thought was the solution: Replace the existing ATS units and their associated switchboards with new low-voltage power circuit breaker switchboards using an interlocked-breaker automatic transfer scheme. (An interlocked breaker consists of two circuit breakers interlocked so only one can close at a time.) (Fig. 2) The interlocked-breakers would transfer the critical load from utility power to the standby generators.

Still a problem: This solution required additional space.

The "final" solution: Use the existing low-voltage power feeder circuit breakers in the existing generator synchboard as one half of each interlocked-breaker. This required the rewiring of each of the existing synchboard feeder breakers to interface with the new switchboard breakers.

How did we arrange this interface? We connected the position contacts on the new breakers to the control circuit of the existing breakers (and vice-versa) to prevent each pair of breakers from closing to the load bus at the same time.

The utility voltage sensing relays and transfer control devices for each interlocked-breaker are installed in a control compartment in each respective new switchboard. The interlocked breakers in Switchboards B and C (these power various low-priority building loads) operate independently.

However, we had to design Switchboards A and D interlocked-breakers to operate in tandem because their load-side busses each feed the new UPS input switchboard. A no-break transfer happens when the preferred feeder breaker remains closed while the alternate breaker is closed to the same bus and then the preferred breaker is opened. We wanted a no-break transfer of the UPS input switchboard from the preferred feeder (Switchboard D) to the alternate feeder (Switchboard A). Such an arrangement would allow the owner to do maintenance work on the preferred feeder breaker without interrupting power to the UPS units. This transfer requires both the preferred and alternate feeder sources either be synchronized together (during transfer) or have the same source voltage. With the interlocked-breakers for Switchboards A and D operating in tandem, the preferred and alternate feeder breakers always have the same source voltage.

We still had to replace the existing switchboards while keeping the facility in full operation. FSSC was very concerned about shutdowns of power to the input of the UPS system, since it could cause a loss of power to the data center. FSSC would allow controlled shutdowns, but only on Sunday nights and only for a few hours. Obviously, there wouldn't be enough time during one shutdown to disconnect the existing feeders, remove the existing switchboards, install the new switchboards, test the new systems, and then reconnect the existing feeders. We had to replace the switchboards in increments.

To allow the contractor to transfer only a few feeders at a time during one brief shutdown (limiting the exposure to the data center), we decided the new switchboards be installed adjacent to the existing switchboards, and then be paralleled together both on the normal power and standby power sides. (See Fig. 3). This approach allowed us to install, test, and put the new switchboards and transfer schemes into operation before connecting critical loads.

Since this paralleling procedure and the switchboard replacement method was somewhat complicated, we developed a "Sequence Of Construction Instructions" to help communicate our thinking to the contractor and FSSC. This planned sequence required the existing switchboards to remain in service during the installation of the new switchboards.

After we placed the new switchboards into service and paralleled them with the existing switchboards, we could relocate the existing feeders. Then we could remove the paralleling connections along with the existing switchboards.

We gave the following note and the "Sequence Of Construction Instructions" to the contractor as part of the construction documents to help come up with a construction schedule.

"The existing facility has a computer system and support infrastructure that cannot tolerate unplanned shutdowns of power. We planned all shutdowns required for tie-in of temporary parallel connections, replacement of existing switchboards, feeder cut overs and similar work, on weekends in close coordination with the owner. See drawing plans, details and specifications for detailed scope of work."